Those are some great ideas, Manuk. You could even combine them and have ethereal devils as your vanguard
I know Aby cant cast it but my #1 spell vs big targets is always Gifts from Heaven. Blasts big creatures to bits in a single hit.
Isnt there a blood spell that diseases an army? Rain of Toads, I think? Now, that is pretty useless vs a normal dom3 300 unit horde. But if all you are facing is a 40 unit army? Then it starts to make a difference.
The suggestion of an SC is a good one, too. Its not that hard to get an SC with truly heroic defense (25+). Which, against giants should be pretty good, given that you cant be mobbed in the traditional sense. Heart finding swords is also and excellent suggestion. In fact, if the giants dont have magic weapons (i dont think they do?) you could make an SC with a heart finder sword, etherealness, lucky, and piles of defense (dont bother with Prot, you will die in one hit anyway) and he would probably kick the hell out of a large number of giants.